Basic mechanisms of tunnel propagation

The basic mechanisms of tunnel propagation and of underground radio systems are briefly reviewed. The TEM approximation of the monofilar mode is compared with available rigorous solutions. Below the tunnel cutoff frequencies, the leaky feeder techniques can be described by the coupled line methods or, with a simpler physical picture, by the use of two eigenmodes. The only valid explanation of the leaky feeder radiation at higher frequencies is the random diffraction of the feeder leakage fields by the wall irregularities. To avoid the inefficiency of this process, the insertion of leaky stubs in a well‐shielded coaxial cable is proposed.